This invention relates generally to automatic focusing systems for cameras. The invention is particularly directed to such systems wherein the camera lens is automatically focused in response to signals generated by a pair of optical systems, one stationary optical system and one scanning optical system.
Conventionally, automatic focusing systems of the type under consideration utilize a scanning mirror in one optical path for scanning a scene from a minimum focusing position to a maximum focusing position. The light from the scanning mirror is directed to an array of photosensitive devices which develop an electrical signal representative of the level of light impinging thereon.
Another non-scanning or stationary optical path is parallel to the optical path of the objective lens and projects light from the subject onto a second array of photosensitive devices. When the outputs of both arrays are in coincidence, a focusing pulse is generated by conventional circuitry. In addition, a control signal is generated when the scanning mirror is at a given position, as at the minimum focusing position for example. By determining the interval between the generation of the control signal and the focusing pulse, a further signal can be derived to drive the lens close to its correct focus position.
A problem associated with the system described above is that the interval between the focusing pulse and the control signal may vary, depending on the brightness of the subject. For example, when the subject reflects a low light level, the response time of the photosensitive arrays is relatively long. Conversely, when the subject reflects a higher light level, the response time of the photosensitive arrays is shorter. Hence, the point in time at which the focusing pulse is generated depends on the level of light reflected from the subject, that is, the brightness of the scene.
Because the time at which the control signal is developed is independent of scene brightness, the interval between the control signal and the focusing pulse may vary and result in inconsistent focusing of the lens.